(a) Field of the Invention
The present invention relates to a porcelain-enameled steel sheet which is excellent, in particular, in porcelain enamel-adhesive properties and which can be used as a marker board, a chalk board, an interior material for tunnel and construction and an exterior finishing material for construction and in particular to a porcelain-enameled steel sheet excellent in the enamel-adhesive properties during and after bending which, in turn, permits the use of the sheet as an interior material for construction after preliminarily burning the porcelain enamel coat and then processing and shaping, chemical durability (chemical resistance), corrosion resistance (end face-rusting inhibitory properties) and glossiness. The present invention also relates to porcelain enamel frits for steel sheets which can be burnt at a low temperature and can impart the foregoing characteristic properties to a porcelain-enameled product after burning.
(b) Description of the Prior Art
There have been known painting and porcelain-enameling methods, as means for imparting various functions such as corrosion resistance, chemical resistance, weatherability and decorative effects to a steel sheet and for improving the lifetime thereof. A porcelain-enameled steel sheet is excellent in, for instance, hardness, weatherability, recovery properties by cleaning, heat resistance and incombustibility as compared with the painted steel sheet, but suffers from a problem in that the forming product is expensive and it may cause peeling of the enamel layer. Therefore, the porcelain-enameled product is in general a so-called post-coated article produced by processing a steel sheet and then porcelain-enameling and the production cost thereof is very high as compared with the so-called pre-coated article which is produced by continuously coil-coating a steel sheet and then subjecting the sheet to fabrication after porcelain-enameling and burning. On the other hand, the film of the painted steel sheet is tolerant, to some extent, to post-processing. Therefore, lightly processed articles may be produced from a pre-coated steel sheet, the production cost can thus be reduced and accordingly, the painted steel sheets have widely been used. In other words, the difference in price between the enameled and painted articles would prevent the expansion of the fields of applications of the enameling technique. However, enameled articles have widely been used in, for instance, hospitals because of their high contamination-inhibitory effects and chemical resistance due to the excellent recovery properties by cleaning. For this reason, there has been desired for the development of a porcelain-enameled steel sheet whose enamel layer is never peeled off even when it is subjected to working through bending after burning the enamel coat as an interior material, in particular, for buildings represented by hospitals wherein sanitation must be considered seriously. In addition, there has also been desired for the development of a porcelain-enameled steel sheet exhibiting resistance to acid rain, i.e., acid resistance and thus capable of being used as an exterior material.
The thickness of the enameled steel sheet is in general not less than 0.3 mm and therefore, the enameled surface thereof undergoes a tensile elongation of not less than 10% during working through bending. The enamel layer is a vitreous one whose rate of elongation is low and thus cannot be saved from any cracking of the enamel layer. Therefore, the enameled steel sheet can withstand working through bending only when it satisfies such essential requirements that the enamel layer is firmly adhered to the steel sheet even if the layer is cracked and that the cracked portions do not gather any rust at all.
Moreover, the baking temperature (burning temperature) for the conventional enamel layer is high on the order of 800 to 900.degree. C. and therefore, the following problems arise: (1) any desired dimensional stability of a product cannot be ensured because of a large extent of heat-deformation when a porcelain enamel film is burnt onto a thin steel sheet and (2) when a plated steel sheet is used as a substrate, the plated layer is melted and flows down because of such a high baking temperature.
Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") No. Hei 6-43257 proposes a porcelain-enameled steel sheet capable of being processed through bending after the enamel coating. This sheet is an enameled steel sheet comprising a metal sheet whose surface portion consists of aluminum and which is provided thereon with a porcelain enamel coating layer (hereinafter simply referred to as "enamel layer") whose composition and thickness are specified. If the enameled steel sheet is subjected to severe processing through bending like the processing applied to the painted steel sheet, however, the cracks of the enamel layer arrive even at the underlying aluminum-plating layer although the enamel layer is not fallen off and the plating layer is thus cracked. The aluminum is excellent in corrosion resistance in itself, but in general it does not show any sacrificial corrosion resistance with regard to iron in the air and thus a problem of red rust formation arises as an inevitable consequence, when the plating layer is cracked. In case of processed enameled steel sheets, the steel material is naturally exposed to the air on the end faces and accordingly, the steel sheets suffer from a problem in that the end faces inevitably gather red rust if the underlying plating comprises aluminum.
On the other hand, J.P. KOKOKU No. Hei 5-71667 discloses an enameled steel sheet provided with a layer of a corrosion-resistant material as an undercoat for the enamel layer. This sheet is an enameled steel sheet wherein the enamel layer is applied onto a plated layer of an aluminum-zinc alloy and this steel sheet inhibits the generation of any red rust gathered at, for instance, pinhole portions on the enamel layer by coating, with an enamel, a plated layer containing zinc which has sacrificial corrosion resistance. However, the zinc component present in the aluminum-zinc alloy plating layer is insufficient in the adhesion to the enamel layer and thus the steel sheet suffers from an essential problem such that the enamel layer is peeled off when the sheet is processed through bending after burning the enamel layer. For this reason, any product (pre-coated article) obtained by post-processing, through bending, of a preliminarily enameled steel sheet cannot be produced from the enameled steel sheet of this patent. More specifically, any enameled steel sheet usable in the post-processing through bending cannot be obtained, at all, by carrying out the method disclosed in J.P. KOKOKU No. Hei 5-71667 by itself. Moreover, in this patent, the composition of the porcelain enamel is not fully investigated and accordingly, the resulting enamel layer does not have satisfactory adhesive properties. In addition, the patent does not fully investigate factors affecting the adhesion between the plated layer and the enamel layer. In this respect, the reason why the adhesion of zinc to the enamel layer is insufficient has not yet been clearly elucidated, but J.P. KOKOKU No. Hei 5-71667 discloses as follows: it would be assumed that zinc is severely oxidized with an inorganic oxide (enamel) because of high susceptibility of zinc to oxidation, this accordingly makes, brittle, the reactive layer at the boundary which would contribute to the adhesion and thus the steel sheet of this patent suffers from a problem of the adhesion between the plated layer and the enamel layer and the resistance of the enamel layer per se to peeling off during processing.
Incidentally, the conventional frits for enameling are insufficient in chemical resistance required for porcelain-enameled articles such as acid resistance, alkali resistance and resistance to weathering due to the reaction with moisture. Moreover, these frits suffer from a problem in that they require the use of a relatively high burning temperature.
For instance, J.P. KOKOKU Nos. Hei 6-43256 and Hei 6-43257 propose an enameled article, i.e., an aluminum plated steel sheet produced by burning at a relatively low temperature ranging from 500 to 540.degree. C. using, as frits for enameling, P.sub.2 O.sub.5 type one which are suitable for low temperature burning.
The foregoing frits for enameling can be burnt at a low temperature, but they are likewise insufficient in chemical resistance required for porcelain-enameled articles, such as acid resistance, alkali resistance and resistance to weathering due to the reaction with moisture.